Central locking woes :-(

[THEHOOD]

Hi, hope someone can help me, the central locking has stopped working on my 89 325i touring, locked it one day with the key to the deadlock position went back the next day and only the drivers door unlocked. But now once the drivers door is unlocked I cannot lock it again with the key, I have to then climb in through the boot and push the black knob down, climb out, lock boot!!

I've read through the wiki and have tried the red/black wire mod to no avail, taken the central locking relay to bits and cannot see anything wrong visually.

Is there anything else I can try/look at?

Thanks in advance.

[Brianmoooore]

Connect a 12 volt bulb of at least 5W across the red/black and brown wires at the locking ECU plug.

[THEHOOD]

Connected a bulb, didn't light up. Will grab my multimeter from work tomorrow and double check.

[Brianmoooore]

Meter will give very misleading results. Bulb is the best tool for the job.
If the bulb didn't light, then it's very likely either fuse 27 is blown or you haven't done the red/black wire mod. correctly.

[THEHOOD]

Ok, il double check things tomorrow after work and post up my results, many thanks mr mooooore

[BenHar]

If the door won't lock with the key, isn't it more likely that the lever on the lock barrel has broken?

Ben

[Brianmoooore]

If the door won't lock with the key, isn't it more likely that the lever on the lock barrel has broken?

Ben

When the central locking fails, the lock mechanism in the driver's door loses its 'power assistance', and after twenty plus years of wear and (probably) zero lubrication, it's very common for the mechanism to be just too stiff to be operated solely by the key.
A breakage of the arm on the back of the barrel may well soon happen if the issue isn't addressed though.

[THEHOOD]

Ok, managed to get back outside, as the weather had a change today, got power at the relay but nothing happens on the relay, no clicks, movement, or anything!! Does that sound like the relay is duff???

Thanks in advance.

[Brianmoooore]

Locking ECU has a thermal overload fuse in one corner of the circuit board, consisting of a small resistor with metal end caps that has one end of a small leaf spring soldered to an end cap with low melting point solder. The current taken by the locking system passes through this resistor, making it become warm. If the current is high enough and maintained for long enough, the resistor gets hot enough to melt the solder, and the leaf springs away, cutting off the current.

[THEHOOD]

Looking at the components on the locking relay, there seems to be no dry joints, signs of burning or overheating, and the thermal resistor is still intact and has not sprung open. There are no signs of water ingress either. I'm at bit lost as to what to check next, I don't want to buy a new relay just to find it still does not work. Any help is most appreciated.

[Brianmoooore]

There are two relays inside the ECU. One sets the locks and the other unlocks them. Try pushing each relay for a quarter of a second in turn to see if this produces any activity.
Locks still might not actually move if they are deadlocked, but it should give some idea as to whether power is reaching them.
Next stage after this will be to simulate the ECU with wire links across the pins of its plug.

[THEHOOD]

Tried yesterday pushing the relay contacts in but it didn't seem to do anything, no movement or even the hint of something happening, what pins can I link to see if anything happens?

[Brianmoooore]

Are you 100% certain that a 12 volt bulb of a least a few watts lights up to full brightness when connected between the red/black and brown wires of the ECU plug?